1. Field of the Invention
The present invention relates to a circuit board for an ink jet head that ejects ink for printing, a method of manufacturing the circuit board, and an ink jet head using the circuit board.
2. Description of the Related Art
An ink jet printing system has an advantage of low running cost because an ink jet head as a printing means can easily be reduced in size, print a high-resolution image at high speed and even form an image on so-called plain paper that is not given any particular treatment. Other advantages include low noise that is achieved by a non-impact printing system employed by the print head and an ability of the print head to easily perform color printing using multiple color inks.
There are a variety of ejection methods available for the ink jet head to realize the ink jet printing system. Among others, ink jet heads using thermal energy to eject ink, such as those disclosed in U.S. Pat. Nos. 4,723,129 and 4,740,796, generally have a construction in which a plurality of heaters to heat ink to generate a bubble in ink and wires for heater electrical connection are formed in one and the same substrate to fabricate an ink jet head circuit board and in which ink ejection nozzles are formed in the circuit board over their associated heaters. This construction allows for easy and high-precision manufacture, through a process similar to a semiconductor fabrication process, of an ink jet head circuit board incorporating a large number of heaters and wires at high density. This helps to realize higher print resolution and faster printing speed, which in turn contributes to a further reduction in size of the ink jet head and a printing apparatus using it.
FIG. 1 and FIG. 2 are a schematic plan view of a heater in a general ink jet head circuit board and a cross-sectional view taken along the line II-II of FIG. 1. As shown in FIG. 2, on a substrate 120 is formed a resistor layer 107 as a lower layer, over which an electrode wire layer 103 is formed as an upper layer. A part of the electrode wire layer 103 is removed to expose the resistor layer 107 to form a heater 102. Electrode wire patterns 205, 207 are wired on the substrate 120 and connected to a drive element circuit and external power supply terminals for supply of electricity from outside. The resistor layer 107 is formed of a material with high electric resistance. Supplying an electric current from outside to the electrode wire layer 103 causes the heater 102, a portion where no electrode wire layer 103 exists, to generate heat energy creating a bubble in ink. Materials of the electrode wire layer 103 mainly include aluminum or aluminum alloy.
In such an ink jet head circuit board, the heater 102 is placed in an onerous environment in which it is subjected to a temperature rise and fall of about 1,000° C. in as little as 0.1-10 microseconds, to mechanical impacts caused by cavitations from repeated creation and collapse of bubbles, and also to erosion. For protection and insulation from ink, the heater 102 is provided with a protective insulation layer 108. This protective insulation layer is required to exhibit good performance in heat resistance, liquid resistance, liquid ingress prevention capability, oxidation stability, insulation, scratch or breakage resistance, and thermal conductivity, and is generally formed of inorganic compounds such as SiO and SiN. Further, because the single protective insulation layer alone may not be able to offer a sufficient protection of the resistor layer, there are cases where a layer of a more mechanically stable metal (e.g., Ta; this layer is generally called an anticavitation layer because of its capability to withstand damages from cavitations) is formed over the protective insulation layer 108 of SiO or SiN (see FIG. 2). In addition to the heater 102, the similar construction for preventing corrosions by ink is also provided for an electrode wire layer 103, which is used to make an electrical connection with a resistor layer 107.
The construction of these protective layers on the ink jet head circuit board constitutes an important factor that determines the performance of the ink jet head, such as its power consumption and service life.
In the construction of the conventional protective layer, however, reducing the power consumption and increasing the reliability of the layer and therefore its longevity are contradictory requirements.
For example, as the thickness of a layer between the heater resistor and a surface in contact with ink decreases, a heat conduction improves and the amount of heat escaping to other than ink decreases, reducing power consumption required to create bubbles. That is, the smaller the effective thickness of the protective layer deposited over the heater resistor, the better the energy efficiency. If on the other hand the protective layer is too thin, pin holes may be formed in the protective layer to expose the heater resistor or the protective layer may not be able to fully cover stepped portions of wires. As a result, ink may infiltrate through these insufficiently covered stepped portions, causing corrosions of wires and heater resistors, which in turn results in degraded reliability and shorter life span.
To deal with these problems, Japanese Patent No. 3382424 proposes a construction using first and second protective insulation layer, in which the first protective insulation layer is removed from above heaters to enhance energy efficiency, lower power consumption and increase reliability of the protective layers as a whole thereby prolonging their longevity.
FIG. 3 is a schematic cross-sectional view of a heater in an ink jet head circuit board disclosed in Japanese Patent No. 3382424 with a cross-sectioned portion corresponding to the line II-II of FIG. 1. In this construction, a first protective insulation layer 108a and a second protective insulation layer 108b are formed over the electrode wire layer 103 and the first protective insulation layer 108a, which is the lower layer, is removed from above the heater 102. This construction reduces the effective thickness of the protective layer over the heater 102 to improve the energy efficiency while at the same time providing a required protective insulation function by the second protective insulation layer 108b. Here, in order to fully cover stepped portions at those ends of the electrode wire layer 103 which face the heater 102, the first protective insulation layer 108a is removed from an area whose boundary is shifted inwardly of the heater from the ends of the electrode wire layer 103.
As ink jet printers are becoming more common in recent years, there are growing demands for higher printing resolution, higher image quality and faster printing speed. Of these demands, the high resolution and high image quality may be met, for example, by reducing the amount of ink ejected for one dot (reducing a diameter of an ink droplet when ink is ejected as a droplet). Conventional practice to achieve a reduction in the volume of ink ejected involves changing the shape of the nozzle (to reduce an orifice area) and reducing an area of each heaters.
It is known that although the heater is heated over its entire surface, a bubble is generated only in a central area 105 excluding a peripheral area, the peripheral area ranging from the edge of the heater to several micrometers inside, because a greater quantity of heat escapes from the periphery. This central area 105 is called an effective bubble generation area.
FIG. 4 shows this mechanism. In FIG. 4 a heater H almost square in plan view is shown connected to the electrode wire E. The peripheral portion N does not contribute to bubble formation and a central area, excluding the peripheral area ranging from the edge to a few micrometers inside, constitutes the effective bubble generation area. As can be seen from this figure, the greater the ratio of the effective bubble generation area A to the entire area of the heater H, the better the heat efficiency is.
FIG. 5 is a graph showing a relation between the size of the heater and a heat efficiency. The area not contributing to the bubble generation, or peripheral portion of the heater, has almost constant width irrespective of the area of the heater (normally 2-3 μm). So, as is seen from this diagram, as the area of the heater decreases to minimize the volume of ink ejected, the heat efficiency decreases.
Thus, if the construction disclosed in Japanese Patent No. 3382424 is adopted, the first protective insulation layer 108a is removed from an area whose boundary is shifted inwardly of the heater 102 from those ends of the electrode wire layer 103 facing the heater. In other words, the first protective insulation layer 108a lies up to a position inside the heater. As a result, the actual bubble generation area further decreases, degrading the heat efficiency. That is, in a present situation calling for reduced areas of the heaters, if the technique disclosed in Japanese Patent No. 3382424 is adopted as is, there is a problem of further degrading the heat efficiency.